The present invention relates to an improved method for preparing photosensitive microcapsules.
U.S. Pat. Nos. 4,399,209 and 4,440,846 disclose imaging systems and processes which employ photosensitive microcapsules. These microcapsules typically contain a photohardenable or photosoftenable composition and a color precursor. The release of the color precursor from the microcapsules is controlled by exposure of the microcapsules to actinic radiation.
Particularly in photohardenable microcapsules which harden by free radical polymerization, it has been found desirable to include a thiol in the photohardenable composition to improve sensitivity. The thiol reacts with oxygen, which inhibits free radical polymerization, in an autoxidative process and may also react via a chain transfer mechanism.
While it has been shown that the thiol improves the sensitivity of photohardenable compositions, the results which have been obtained when such thiol-containing photohardenable compositions are microencapsulated have been somewhat mixed and difficult to reproduce. The improvements in sensitivity which have been observed for microcapsules have not been as large as those observed for unencapsulated compositions. This observation has lead to the discovery that the microencapsulation process is interfering with the incorporation of the thiol into the photohardenable composition.
In microencapsulating photohardenable compositions, an oil-in-water emulsion is prepared by emulsifying the constituents of the photohardenable composition in an aqueous medium and building the microcapsule wall around the emulsified droplets of the internal phase. In forming the wall, it is not unusual to effect one or more pH changes in the aqueous phase. In particular, in the preparation of urea-formaldehyde and melamine formaldehyde microcapsules, after the wall is formed, the pH is adjusted to about 9.5 to scavenge excess formaldehyde. It is believed that at the alkaline pH, the thiol is deprotonated and extracted from the photohardenable composition into the aqueous phase. As a result, the concentration of the thiol in the photohardenable composition is reduced and the thiol is not as effective in improving sensitivity.
Accordingly, there is a need for improved methods for forming photosensitive microcapsules containing thiols.